As a conventional automatic focus adjustment operation used in a video equipment such as a video camera or the like, a so-called “hill-climbing operation” is known. In this operation, a high-frequency component in an image signal obtained from an image sensing element such as a CCD is extracted, and focus adjustment is done by driving a focusing lens to maximize the high-frequency component. Since the focus adjustment system based on this “hill-climbing operation” makes focus detection based on the sharpness of an object image, it can accurately adjust the focus irrespective of the object distance, i.e., even when an object is located at a far or closest distance position.
As conventional auto-focusing (to be abbreviated to “AF” hereinafter) based on the hill-climbing operation, an integration AF method that integrates, within a predetermined distance measurement range of a frame, image signal components which have undergone a high-pass filter process for extracting a high-frequency component and uses an integrated signal in focusing control is prevalently used. Since the integration AF method integrates extracted high-frequency components within the distance measurement range and uses the integrated signal, it is excellent in AF stability, and can easily obtain an optimal in-focus point.
FIG. 14 is a block diagram showing an arrangement of a video camera system adopting the conventional integration AF method.
Referring to FIG. 14, reference numeral 101 denotes a stationary first lens group; 102, a zoom lens for zooming; 103, an iris for adjusting the quantity of light; and 104, a stationary second lens group. Reference numeral 105 denotes a focus compensation lens (to be referred to as a “focus lens” hereinafter) which has a function of compensating for movement of a focal plane upon zooming, and a focus adjustment function.
Reference numeral 106 denotes a CCD as an image sensing element. Reference numeral 107 denotes a CDS/AGC which performs sample-and-hold operation and amplifies the output from the CCD 106, and the gain of which is adjusted by a signal from a camera controller (microcomputer) 114 to be described later. Reference numeral 108 denotes an A/D converter for converting an analog signal from the CDS/AGC 107 into a digital signal; and 109, a camera signal processing circuit. The output signal from the camera signal processing circuit 109 is recorded on a recording medium such as a magnetic tape, memory, or the like (not shown).
Reference numeral 110 denotes a timing signal generator for supplying various drive pulses and timing pulses to the respective sections of the camera, such as the CCD 106, CDS/AGC 107, and the like.
Reference numeral 111 denotes a zoom lens driver for driving the zoom lens 102; 112, an iris driver for driving the iris 103; and 113, a focus lens driver for driving the focus lens 105. The drivers 111 to 113 respectively drive motors included therein in accordance with signals from the camera controller 114.
Reference numeral 120 denotes a focus evaluation value processor, which has a high-pass filter 121 for extracting a predetermined high-frequency component from a luminance signal output from the A/D converter 108, a distance measurement range gate 122 for extracting only signal components within a predetermined distance measurement range from a frame, a line peak hold circuit 123 for holding peaks of the signal components extracted by the distance measurement range gate 122 in units of horizontal scan lines, and an integrator 124 for integrating peak values of the peak-held scan lines. The integrated value is called an “integrated focus evaluation value”.
The value of the integrator 124 is reset for each frame, and the integrator 124 can compute the integrated focus evaluation value for each frame. The integrated focus evaluation value is input to the camera controller 114, which drives the focus lens 105 via the focus lens driver 113 to maximize the integrated focus evaluation value.
In the prior art, the peak values of the peak-held scan lines are integrated. Alternatively, the signal components may be integrated without holding peaks, or peak values may be held in units of a plurality of horizontal scan lines, and the held values may be integrated.
The output luminance signal from the A/D converter 108 is also input to an exposure evaluation value processor 130, which generates an evaluation value for controlling exposure from signal components of a frame and supplies it to the camera controller 114. The camera controller 114 drives the iris 103 based on the exposure evaluation value to obtain an optimal exposure value. A key unit 115 is connected to the camera controller 114, and various kinds of key operation information of the camera unit such as a zoom key for operating the zoom lens 102 and the like are output to the camera controller 114. For example, when the user has pressed the zoom key, the camera controller 114 drives the zoom lens 102 via the zoom lens driver 111 to obtain a desired zoom ratio (focal length).
However, since the integration AF method controls to obtain the highest average contrast within a frame, although no problem is posed for a normal object focusing performance impairs for an image of a high-luminance object or a point light source in a night scene (to be referred to as a “peak image” hereinafter).
FIG. 15 shows an example of the integrated focus evaluation value of a normal object, and FIG. 16 shows an example of the integrated focus evaluation value of a peak image. In the normal object, since an in-focus point and a peak value of the integrated focus evaluation value appear at an identical position P0, as shown in FIG. 15, and no problem is posed. However, in the peak image, an in-focus point and a peak of the integrated focus evaluation value do not appear at the same position, and the peak of the integrated focus evaluation value appears at a slightly offset focus lens position PB, as shown in FIG. 16.
In case of an image of a point light source in a night scene as a typical peak image, the integrated focus evaluation value assumes a larger value in an out-of-focus state shown in FIG. 18 than a perfect in-focus state shown in FIG. 17. Note that FIGS. 17 and 18 show examples of images that appear on the frame upon sensing a peak image. In this manner, a peak of the integrated focus evaluation value appears at a focus lens position offset from an actual in-focus point. In such case, since the camera controller 114 controls the focus lens 105 to a position where the integrated focus evaluation value has a peak, the focus lens 105 is controlled to the focus lens position PB shown in FIG. 16. As a result, the obtained image is out of focus.
As an alternative to the integration AF method, a peak AF method that uses a maximum peak value within a predetermined distance measurement range from luminance signal components of a frame that has undergone a high-pass filter process for extracting high-frequency components is known. FIG. 19 is a block diagram showing the arrangement of a video camera system that adopts the conventional peak AF method.
The video camera shown in FIG. 19 has substantially the same arrangement as that of the video camera shown in FIG. 14, except that the arrangement of the focus evaluation value processor 120 is modified to obtain the maximum value of high-frequency components of the luminance signal within the distance measurement range. Referring to FIG. 19, the maximum value of high-frequency components of the luminance signal within the distance measurement range, which is output from a focus evaluation value processor 120′ is called a “peak focus evaluation value”.
FIG. 20 shows a peak focus evaluation value in a peak image which is hard to focus upon adjustment by integration AF. For the purpose of comparison with integration AF, FIG. 20 also shows the integrated focus evaluation value. As can be seen from FIG. 20, the peak focus evaluation value becomes maximal at a correct in-focus point even in a peak image. FIGS. 21A and 21B respectively show a luminance signal for one horizontal line of a portion indicated by LA in FIG. 17, and the output of a high-pass filter of an in-focus image of a point light source. FIGS. 22A and 22B respectively show a luminance signal for one horizontal line of a portion indicated by LB in FIG. 18, and the output of a high-pass filter of an out-of-focus image of a point light source.
In this manner, even in a peak image with a saturated luminance signal, since in-focus and out-of-focus images have different leading edges of luminance signals, the output from the high-pass filter differs in correspondence with these images, thus allowing correct in-focus point detecting. In the peak AF method, a correct in-focus point can be detected in a peak image.
However, since the peak focus evaluation value is smaller than the integrated focus evaluation value, the peak AF method has poor stability for a normal object compared to the integration AF method, and AF often becomes unstable under the influence of, e.g., panning.
Since it is controlled to obtain the highest average contrast within a frame in the integration AF method, no problem is posed in a normal object, but focusing performance impairs in a peak image.